1. Field of the Invention
The invention relates to the field of MOS processing.
2. Prior Art
In the fabrication of metal-oxide-semiconductor (MOS) integrated circuits, particularly in processes employing polysilicon, a glass layer is typically formed over the polysilicon members. This glass layer is generally a doped silicon dioxide used to protect the underlying circuit members. Openings are formed in this glass layer to permit overlying metal members to contact either substrate regions or polysilicon members. To provide good metal step coverage on the glass layer, the glass layer is generally subjected to a relatively high temperature causing the glass to reflow, thereby rounding sharp corners. This reflowing step is performed prior to any metallization, since the temperatures associated with the reflowing will cause problems such as oxidation or the formation of silicides (in the case of refractory metals) or melting (in the case of aluminum).
This high temperature glass reflowing step prevents the formation of metal members below the glass layer, and thus typically polysilicon, not metal members are formed below the glass layer. Polysilicon, even when highly doped, has a high resistance when compared to metal. This higher resistance reduces the speed of MOS circuits.
Typical MOS processes using glass layers are discussed in U.S. Pat. Nos. 4,033,026 and 4,052,229.
As will be seen, the present invention permits formation of metal members in contact with the polysilicon members thereby reducing the resistance of the polysilicon members. Low temperature "rear end" processing is used to prevent damage to the underlying metal. ("Rear end" processing refers to that part of the MOS processing after patterning of the gates and doping of the source and drain regions and other steps before the application of a low temperature protective layer).